1. Field of the Invention
This invention relates to scanning probe microscopes (SPMs) and other related metrology apparatus. More particularly, it is directed to apparatus and methods for measuring the movement of a probe of, for example, an SPM, and minimizing the negative effects associated with parasitic movement of such a probe.
2. Discussion of the Prior Art
Scanning probe microscopes are typically used to determine the surface characteristics of a sample, commonly biological or semiconductor samples, to a high degree of accuracy, down to the xc3x85ngstrom scale. Two common forms of the scanning probe microscope are shown in FIGS. 1A and 1B. A scanning probe microscope operates by scanning a measuring probe assembly having a sharp stylus over a sample surface while measuring one or more properties of the surface. The examples shown in FIGS. 1A and 1B are atomic force microscopes (xe2x80x9cAFMsxe2x80x9d) where a measuring probe assembly 12 includes a sharp stylus 14 attached to a flexible cantilever 16. Commonly, an actuator such as a piezoelectric tube (often referred to hereinafter as a xe2x80x9cpiezo tubexe2x80x9d) is used to generate relative motion between the measuring probe 12 and the sample surface. A piezoelectric tube is a device that moves in one or more directions when voltages are applied to electrodes disposed inside and outside the tube (29 in FIG. 1C).
In FIG. 1A, measuring probe assembly 12 is attached to a piezoelectric tube actuator 18 so that the probe may be scanned over a sample 20 fixed to a support 22. FIG. 1B shows an alternative embodiment where the probe assembly 12 is held in place and the sample 20, which is coupled to a piezoelectric tube actuator 24, is scanned under it. In both AFM examples in FIGS. 1A and 1B, the deflection of the cantilever 16 is measured by reflecting a laser beam 26 off the back side of cantilever 16 and towards a position sensitive detector 28.
One of the continuing concerns with these devices is how to improve their accuracy. Since these microscopes often measure surface characteristics on the order of xc3x85ngstroms, positioning the sample and probe with respect to each other is critical. Referring to FIG. 1C, as implemented in the arrangement of FIG. 1A, when an appropriate voltage (Vx or Vy) is applied to electrodes 29 disposed on the upper portion 30 of piezoelectric tube actuator 18, called an X and Y-axis translating section or more commonly an xe2x80x9cX-Y tube,xe2x80x9d the upper portion may bend in two axes, the X and Y-axes as shown. When a voltage (Vz) is applied across electrodes 29 in the lower portion 32 of tube 18, called a Z-axis translating section or more commonly a xe2x80x9cZ-tube,xe2x80x9d the lower portion extends or retracts, generally vertically. In this manner, portions 30, 32 and the probe (or sample) can be steered left or right, forward or backward and up and down. This arrangement provides three degrees of freedom of motion. For the arrangement illustrated in FIG. 1A, with one end fixed to a microscope frame (for example, 34 in FIG. 1D), the free end of tube 18 can be moved in three orthogonal directions with relation to the sample 20.
Unfortunately, piezoelectric tubes and other types of actuators are imperfect. For example, the piezo tube often does not move only in the intended direction. FIG. 1D shows an undesirable, yet common, case where a piezo tube actuator 18 was commanded to move in the Z-direction (by the application of an appropriate voltage between the inner and outer electrodes, 29 in FIG. 1C), but where, in response, the Z-tube 18 moves not only in the Z-direction, but in the X and/or Y-directions as well. This unwanted parasitic motion, shown in FIG. 1D as xcex94X, limits the accuracy of measurements obtained by scanning probe microscopes. Similar parasitic motion in the Y-direction is also common. The amount of this parasitic motion varies with the geometry of the tube and with the uniformity of the tube material, but typically cannot be eliminated to the accuracy required by present instruments.
Current methods of monitoring the motion of the probe or sample 20 when driven by a piezoelectric tube are not sufficiently developed to compensate for this parasitic X and Y-error. The devices are typically calibrated by applying a voltage to the X-Y tube and the Z-tube, and then measuring the actual distance that the probe travels. Thus, the position of the free end of the piezo tube is estimated by the voltage that is applied to the X-Y tube and the Z-tube. However, because the (X,Y) position error introduced by the Z-tube on the probe (or on the sample for the arrangement shown in FIG. 1B) is essentially random, it cannot be eliminated merely by measuring the voltage applied to the Z-tube or to the X-Y tube.
Moreover, with respect to movement in the intended direction, piezoelectric tubes and other types of actuators typically do not move in a predictable way when known voltages are applied. The ideal behavior would be that the actuator move in exact proportion to the voltage applied. Instead actuators, including piezo tubes, move in a non-linear manner, meaning that their sensitivity (e.g., nanometers of motion per applied voltage) can vary as the voltage increases. In addition, they suffer from hysteresis effects. Most generally, the response to an incremental voltage change will depend on the history of previous voltages applied to the actuator. This hysteresis effect, thus, can cause a large prior motion to affect the response of a commanded move, even many minutes later.
Notably, also, vertical measurements in scanning probe microscopy are typically made by moving the probe up or down in response to the rising or falling sample surface. For example, for AFM operation in tapping mode, the actual vertical measurement is the average distance the probe moves in the vertical direction to maintain a constant oscillation magnitude as it taps the surface, while for AFM operation in contact mode, the vertical measurement is the distance the probe moves to maintain a particular amount of force between the cantilever stylus and the sample surface. This distance is often calculated mathematically by recording the voltage applied to the piezoelectric tube and then multiplying by the tube""s calibrated sensitivity in nm/V. But as mentioned previously, this sensitivity is not constant and depends on the previous voltages applied to the tube. So using the voltage applied to the tube to calculate the vertical motion of the tube will always result in an error with respect to the actual motion. This error can translate directly into errors when measuring surface topography of a sample.
What is needed, therefore, is an apparatus and method for controlling the motion of the probe or sample to minimize the effects due to, for example, adverse parasitic motion introduced by an actuator (e.g., a Z-tube) in a metrology apparatus. Moreover, an apparatus is needed to measure the magnitude of the intended motion to, for example, track whether intended motion is being realized.
The present invention is directed to an apparatus and method for controlling the motion of a metrology probe to minimize negative effects associated with parasitic motion introduced, for example, by an actuator (e.g., a Z-tube) in a metrology apparatus such as an SPM or a profiler. In addition, the apparatus measures the magnitude of the actual motion. Notably, the apparatus implements an optical detection apparatus, utilizing an arrangement of reflecting surfaces disposed in corner-cube retroreflector-like relationship, so that the light reflected and sensed by the apparatus is relatively immune to, for example, lateral deflections of components of the microscope coupled thereto.
According to first aspect of the preferred embodiment, a Z isolating/measuring assembly for a metrology apparatus includes a piezoelectric or electrostrictive actuator assembly including a first actuator stage configured to controllably move in first and second orthogonal directions, and a second actuator stage integrated with or coupled to the first actuator stage, and being configured to controllably move in a third direction orthogonal to the first and second orthogonal directions. In addition, the assembly includes a reference structure having first and second ends wherein the first end is fixed relative to movement of the second actuator stage. The assembly also includes a multi-bar linkage assembly fixed to the second end of the reference structure, and a coupling attached to the second actuator stage and to the multi-bar linkage, wherein the second actuator stage and the coupling move the linkage in the third orthogonal direction in a manner that substantially isolates the linkage from any second actuator stage motion in the first and second directions.
According to another aspect of the invention, a Z isolating/measuring assembly includes an elongate actuator with a longitudinal axis having a fixed end, and a free end configured to translate in three orthogonal directions with respect to the fixed end. In addition, the assembly includes a multiple bar linkage having first and second links mutually constrained to translate with respect to each other, and wherein the first link is fixed to a reference structure and the second link is constrained to translate in a direction parallel to the longitudinal axis of the actuator. The assembly also has a coupling having first and second ends, the first end being fixed to the actuator proximate to its free end, and the second end being fixed to the second link, the coupling adapted to transmit force and therefore displacement in a direction substantially parallel to the longitudinal axis of the actuator.
According to yet another aspect of the invention, a method of reducing positioning errors at the free end of an elongate actuator of a metrology apparatus includes the step of supporting a probe assembly on a probe support assembly. In addition, the method includes supporting the probe support assembly at a first end to a reference structure of the metrology apparatus, the reference structure being substantially insensitive to longitudinal expansion or contraction of the elongate actuator. The method further includes isolating the reference structure from a longitudinal tube deflection of the actuator. When driving the actuator, the metrology apparatus simultaneously generates both longitudinal deflections as well as lateral deflections in the longitudinally expanding and contracting portion. The method operates to prevent the lateral deflections generated in the longitudinally expanding and contracting portion of the tube from laterally deflecting the probe support assembly, while simultaneously transmitting the longitudinal deflections to the probe support assembly.
According to yet another aspect of the preferred embodiment, an apparatus for measuring movement of an actuator in a metrology apparatus such as a scanning probe microscope (SPM) includes an optical measuring device having a light source that generates a light beam, the measuring device being configured to change the direction of the beam in response to movement of the actuator. The apparatus also includes a sensor to detect the beam position and generate a signal indicative of the movement of the actuator.
According to another aspect of this embodiment, the measuring device includes a movable bar assembly coupled to the actuator and to a reference structure, wherein the bar assembly has a reflecting surface that is adapted to deflect the beam. Moreover, the bar assembly is responsive to movement of the actuator so as to change the direction of the deflected beam.
According to a further aspect of the preferred embodiment, a method for measuring movement of an actuator in a metrology apparatus such as a scanning probe microscope (SPM) includes the steps of providing a movable bar assembly coupled to the actuator and to a reference structure. Moreover, the method includes measuring, in response to movement of the actuator, movement of the movable bar assembly, wherein movement of the movable bar assembly is indicative of movement of the actuator.
According to another aspect of the preferred embodiment, an apparatus for ensuring that displacement generated by an actuator, and transferred to the cantilevered probe coupled thereto, is isolated from movement of the metrology apparatus in a direction other than the intended direction associated with the actuator, includes a flexure that is coupled to the actuator via a flexible bar or member (i.e., a coupling). The coupling is adapted to transmit displacement only in an intended direction, thus minimizing adverse affects associated with parasitic movement of at least a portion of the metrology apparatus, such as the actuator. The apparatus also includes a fixed reference structure to which the flexure is also attached. Preferably, the flexure is a parallelogram flexure comprising a four bar linkage that is adapted to translate so that its opposed vertical links remain generally orthogonal to the X-Y plane in response to force and therefore displacement transmitted in the vertical or xe2x80x9cZxe2x80x9d direction by the coupling, this outcome due at least in part to the connection of the flexure to the reference structure.
According to a still further aspect of the preferred embodiment, in a metrology apparatus such as a scanning probe microscope (SPM) having an actuator for moving a probe in a particular direction, a reference assembly that generally decouples movement in a direction other than the particular direction from the probe, the reference assembly including a reference structure and a probe support assembly coupled to the reference structure and to the actuator, wherein the probe is attached to the probe support assembly. Further, the apparatus includes a flexible bar having opposed ends, one of which is coupled to the actuator and the other of which is coupled to the probe support assembly, wherein the flexible bar, the reference structure, and the probe support assembly are adapted to collectively decouple movement of the metrology apparatus in a direction other than the particular direction from the probe.
Moreover, the flexible bar is preferably more flexible in response to displacements applied thereto in any direction other than the particular direction than to a displacement applied in the particular direction.
These and other objects, features, and advantages of the invention will become apparent to those skilled in the art from the following detailed description and the accompanying drawings. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the present invention, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications.